Ideally BSFC would be a crv file if you wanted to provide one, vs the curve.

But then different engines have varying BSFC based on load too, due to types of throttle etc.

Really a BSFC should be a 3d plot with rpm vs load vs the BSFC value.

Sooooo, all in I'd just pick a middling value for now that seems to get you about the right average economy doing what you want to do with the car.

If you want specific economy to be right then script access will be essential, if it exists, and then you can draw yourself a matrix LUT inside the script, say an 8x8 table with linear interpolation would probably be ok.